ad hominem : Java Glossary

The Nizkor
Project explains many of the logical fallacies frequently used in debate. They
define ad hominem this way:

Translated from Latin to English, Ad Hominem means
against the man or against the
person.

An Ad Hominem is a general category of fallacies in which a claim or argument is
rejected on the basis of some irrelevant fact about the author of or the person
presenting the claim or argument. Typically, this fallacy involves two steps. First,
an attack against the character of person making the claim, her circumstances, or her
actions is made (or the character, circumstances, or actions of the person reporting
the claim). Second, this attack is taken to be evidence against the claim or argument
the person in question is making (or presenting). This type of argument has the following form:

Person A makes claim X.

Person B makes an attack on person A.

Therefore A’s claim is false.

The reason why an Ad Hominem (of any kind) is a fallacy is that the character,
circumstances, or actions of a person do not (in most cases) have a bearing on the
truth or falsity of the claim being made (or the quality of the argument being
made).

Closely related to the Ad Hominem is the personal attack.

Person A makes claim X.

Person B can’t think anything to rebut the claim, so he tries to change
the subject by insulting A, hoping this will derail the discussion away from the
embarrassing area. Most A’s will fall for the trap, rushing to defend
themselves.

Unlike a standard Ad Hominem, B makes no claim that A’s original
assertion is thus false because of the insult.